1. Field of the Invention
The present invention relates to a continuous process for extruding polymeric foam having an average transverse cell size of less than one micrometer, preferably less than 500 nanometers.
2. Introduction
Increasing energy efficiency is an ever present goal. One large use of energy is in creating and maintaining environments at a particularly desirable temperature by heating and/or cooling. Efficient use of energy while controlling temperature requires minimizing thermal energy transport between the area of controlled temperature and the environment surrounding that area. Therefore, thermal insulating materials are commonly used to isolate temperature controlled areas from other areas that may be at a different temperature. Thermally insulating materials are commonplace in building structures and appliances such as refrigerators and freezers for instance.
Polymeric foam has long been used as a thermal insulating material. Historically, typical thermally insulating polymeric foam comprises a plurality of closed cells having dimensions of 100 micrometer or greater and require presence of gas having low thermal conductivity in the foam cells. While these polymeric foams serve well as thermally insulating materials, there is opportunity to improve the thermally insulating properties of polymeric foam without resorting to low thermal conductivity gases. One characteristic of polymeric foam that controls thermal conductivity through the foam is the cell size. Behavior of gas molecules in foam cells can contribute to thermal conductivity through the polymer foam if the gas molecules are free to move within the cells and collide with the cell walls. Cell size has little influence on the contribution of cell gas to the thermal conductivity through foam when the cell size is between about one micron and about one millimeter. Convection behavior of a gas within a foam cell tends to increase thermal conductivity through the foam when the cell size exceeds about one millimeter. The contribution of cell gas to thermal conductivity through polymeric foam decreases dramatically when the cell size of the foam is reduced below one micrometer. For example, thermal conductivity due to cell gas reduces almost in half upon reducing a foam cell size from one micrometer to 300 nanometers (nm) and reduces by almost ⅔ upon reducing the cell size from one micrometer to below 100 nm. Therefore, polymeric foam having a nanoporous structure (that is, having an average cell size that is below one micron), especially polymeric foam having an average cell size of 300 nm or less, and most preferably 100 nm or less is desirable as thermal insulation. In particular, it is desirable for the polymeric foam to have such cell size dimensions as measured in a direction of the foam through which thermal conductance occurs (for example, the thickness dimension of a foam board). For extruded foam, for example, this cell size dimension typically corresponds to a dimension in a transverse direction of the foam, which is a direction in a plane perpendicular to the foam's extrusion direction.
It is further desirable for thermally insulating polymeric foam to have a high void volume. Generally, thermal conductivity is higher through the polymer network of a polymeric foam structure than through the cell gas. Therefore, maximizing the amount void space due to cells in foam will generally result in a decrease in thermal conductivity through the foam. This is particularly true for polymeric foam having a nanoporous structure. One way to characterize void volume is by “porosity”, which is the ratio of void volume to foam volume. Porosity values of 0.50 or greater are desirable for thermally insulating foam.
Preparation of polymeric foam having a nanoporous structure (that is, “nanofoam”) in a commercially viable manner has proven challenging, particularly with a high enough void volume to make it a desirable thermally insulating material. Current processes for preparing thermally insulating polymeric foam are typically continuous extrusion processes. Continuous extrusion processes are desirable because they can produce greater quantities of product in less time than, for example, batch processes. Yet, the technology required for preparing nanofoam has proven challenging to incorporate in a continuous extrusion process at least partially due to the amount and type of blowing agent required to prepare nanofoam. Nanofoam has typically been prepared in batch processes using supercritical carbon dioxide (or a similar blowing agent) under extremely high pressures. Few have achieved a continuous extrusion process for producing nanofoam.
U.S. reissue patent 37,932E describes a process for preparing polymeric foam having cell sizes that can be below 0.1 micrometers that includes use of an extruder to prepare a foamable polymer composition and then extrude that composition into a mold. The “extrusion” process is really a batch process since the extruder is only used to fill a mold with foam rather than extrude a continuous foam article. It would be desirable to have a truly continuous process that extrudes a continuous foam article. U.S. Re37,932E discloses a method of imbibing solid polymer with carbon dioxide and a method of blending supercritical carbon dioxide fluid into a molten polymer material. The molten process involves dissolving the carbon dioxide into the molten polymer to form a homogeneous and uniform fluid/polymer solution and then heating the mixture. Heating reduces the carbon dioxide solubility and initiates nucleation of the carbon dioxide blowing agent. However, heating to initiate nucleation is an energy intensive step that would be desirable to avoid.
U.S. Pat. No. 6,383,424 discloses an extrusion process for preparing polymeric membranes and claims such membranes having a microcellular structure of 0.5 to 15 micrometers. The extrusion process requires mixing carbon dioxide with a polymer melt to achieve near complete dissolution of the carbon dioxide into the melt. The process then requires reducing the temperature and increasing the pressure to push the polymer out through a shaping device. The process requires a means (such as a pump) for increasing pressure on a polymer melt after mixing in blowing agent. The step of increasing pressure adds complexity to the process both by requiring additional equipment (for example, an additional pump) and by requiring heavy duty equipment that can withstand the pressures of the process (the reference identifies the pressure is in the range of up to 1500 bars). It is desirable to be able to prepare nanofoam without requiring an increase in pressure after mixing blowing agent with a polymer melt. It is further desirable to be able to achieve cell sizes below 0.45 microns.
U.S. Pat. No. 5,866,053 discloses a process for producing a continuous stream of supermicrocellular polymers. U.S. Pat. No. 5,866,053 teaches that only a soluble amount of carbon dioxide blowing agent can be added to a polymer melt or undesirably voids in the polymer melt will occur, resulting in hollow cavities in the final product. It is desirable, however, to incorporate into a foamable polymer composition more blowing agent than is soluble in the polymer melt in order to lower foam density while at the same time avoiding undesirable voids in the polymer melt and cavities in the final product.
U.S. Pat. No. 7,838,108 discloses theoretical concepts for making nanofoam that include conceptually how to prepare nanofoam by extrusion methods. U.S. Pat. No. 7,838,108 discloses addition of dry ice (solid carbon dioxide) to a polymer melt in combination with carbon dioxide gas in order to achieve a homogeneous phase in a single phase solution zone of the extruder. Combining dry ice with a polymer melt is a challenging process to do safely due to the volatility of the dry ice and the tremendous temperature difference between the dry ice and the polymer melt. Additionally, as with U.S. Pat. No. 5,866,053, adding only enough carbon dioxide to achieve a homogenous phase in a single phase solution restricts the amount of carbon dioxide that can be added in the process to the solubility limit of the polymer melt, which restricts how low of a density is achievable and nascent cell count in the resulting foam.
While processes for preparing nanofoam using an extruder are known, there remains opportunity to improve and advance the technology of producing nanofoam by continuous extrusion. In particular, it is desirable to be able to have a truly continuous extrusion process that produces a continuous foam article as opposed to a process that extrudes foamable compositions into a mold. Moreover, it is desirable to provide a process that includes mixing into a polymer melt more blowing agent than is soluble in the polymer melt in order to achieve low density foam, but do so without creating undesirably large hollow cavities in the final product. It is further desirable for the process to be free from having to mix dry ice with a polymer melt or increase the temperature of or pressure on the polymer melt after introducing blowing agent and prior to extruding.